The present invention has particular, but not exclusive, application in frequency hopping systems operating in the ISM bands in which strict rules apply, including a rule that a transmitter must continuously frequency hop. Although a receiver will store the hopping sequence of N frequencies, where N is greater than 2, for example, 50 or 75, when it switches-on it will not know in advance which one of the hopping frequencies is currently being transmitted. However, once it has detected one hopping frequency in the sequence, the receiver can synchronise with the phase of the sequence and will be able to track the sequence of hopping frequencies using a frequency synthesiser.
Various techniques are known in the art for a receiver to acquire synchronisation. In one technique the receiver tunes to one of the N frequencies and remains on that frequency until hop synchronisation is acquired or N hop periods have elapsed. In the latter event the receiver tries another of the N frequencies. This technique can lead to long acquisition times if the chosen frequency is suffering interference.
U.S. Pat. No. 5,471,503 discloses a frequency hopping system having N frequency channels or hops and assumes error correction techniques which allow a transmission to be recovered if no more than e segments out of the N are corrupted. In implementing receiver acquisition, the receiver continuously scans the first, p, (where 1≦p≦e+1), channels and tests each channel for the existence of a transmission. A scan time for each frequency of the p channels is substantially less than the time to receive a segment. If a transmission is detected then the receiver synchronises itself with the transmitted sequence This cited method requires the receiver to be capable of scanning the p channels at a higher rate than the frequency hopping rate which is not always desirable in communications apparatus.